The present invention relates to crucibles for fusion of a specimen and particularly to crucibles made of a resistive material.
Graphite crucibles are employed in analytical furnaces for fusing specimens to temperatures of about 2500° C. to about 3000° C. with the carbon interacting with the specimen to release gases, which are swept from the furnace area by an inert gas to detectors for subsequent determination of elements such as hydrogen, nitrogen, and oxygen. A current example of such an analytical instrument is an ONH836 furnace and analyzer available from Leco Corporation of St. Joseph, Mich. Such instruments employ a pair of electrodes, which clamp a graphite crucible therebetween and provide up to 1000 or more amps current through the graphite crucible for heating the crucible and specimen to the near 3000° C. temperature.
There have been many crucible designs employed with analytical furnaces over the years, including those described in U.S. Pat. Nos. 3,636,229, 3,899,627, and 4,328,386. Prior electrode construction for mating with such crucibles is described in U.S. Pat. Nos. 3,936,587 and 4,056,677. In order to protect the electrodes from erosion due to the high temperatures, tungsten/copper alloy tips have been employed on the upper and lower electrodes which engage the top annular rim of the crucible and the downwardly projecting pedestal, respectively, on prior art crucibles. A replaceable tip for a lower electrode is disclosed in U.S. Pat. No. 4,419,754, which discloses a removable tungsten/copper alloy tip.
Over the years, improvements have been made to such analytical furnaces, including the introduction of samples into the furnace area through a central opening in the upper electrode, such as represented by the sample drop mechanism disclosed in U.S. Pat. No. 6,291,802. Further, such instruments and furnaces have also been improved by providing automated sample loading mechanisms, such as disclosed in U.S. Pat. No. 7,402,280 and/or the use of a sample loading carousel, as disclosed in pending patent application Ser. No. 13/402,192, entitled SAMPLE LOADING CAROUSEL, which was filed on Feb. 22, 2012, now U.S. Pat. No. 9,188,598. Also, improvements have been made in the cleaning of electrodes between analyses, such that continuous sequential analyses can be undertaken without manual cleaning of the electrodes or electrode components. One such system employed for the automated cleaning of electrodes is describe in U.S. patent application Ser. No. 13/358,096 entitled VACUUM CLEANING STRUCTURE FOR ELECTRODE FURNACE, which was filed on Jan. 25, 2012, now U.S. Pat. No. 9,042,425, the disclosure of which is incorporated herein by reference.
These various improvements to the earlier models of resistance furnaces have resulted in improved performance in terms of accuracy and repeatability of sample analysis. Existing furnaces, however, typically require a significant amount of power, and prior crucibles have suffered with hot spots within the crucible which tends to overheat specimens, causing their bubbling. In some cases, the molten specimens bubble out of the crucible and, in extreme cases, contaminate the upper electrode which is in contact with the open mouth of such crucibles. In order to resolve the problems with electrode wear, crucible hot spots, as well as occasional breakage of a crucible when under compression between the electrodes, an effort has been made to redesign crucibles and provide a crucible which provides even heating characteristics at the floor and lower side walls of the crucible where the specimen rests during fusion, a crucible that requires less power to reach the desired operating temperature, and a crucible which remains economically feasible to manufacture on a large scale basis.